Friction

Friction is resistance to motion between two objects that rub against one another. If a person tries to push a box across the floor, he or she must push hard to overcome the resistance. If the person applies a push, or force that is stronger (larger) than the frictional force, the box will move. If the push isn't strong enough the box won't move.

The friction between two moving objects can be affected by the surfaces of the objects. For example, it is easier to push a heavy box across a smooth wood floor or a sheet of ice than it is to push it across thick, bumpy carpet. That means the friction between the box and the smooth floor or ice is less than the friction between the box and the thick carpet. It takes less of a push to get it moving.

When air flows across a surface like our tennis ball or an airplane's wing, there is friction resisting the motion. How much friction is dependent on two factors, the viscosity of the fluid and the smoothness of the surface.

In the case of the tennis ball, the surface is quite rough as compared to a smooth ball. If you poured a little bit of water on a tennis ball, it will travel down more slowly than on a smooth ball. Because the surface is rougher, the friction force is stronger, the velocity is slower. If instead you poured a little bit of honey on the tennis ball, the honey would flow slower than the water did, because honey is more viscous than water.